If you have a custom domain name for your blog there are two areas that currently will not work as expected:

Previewing draft posts

Using the Appearance > Customize preview part of the website

To work around this restriction:

Temporarily change the primary domain for your blog:

Go to Tools > Domain Mapping

Under “Active domains on this blog“

Change the primary domain to be the original *.blogs.ilrt.org one.

Make changes in the Appearance > Customize area or preview your draft blog posts

Once changes have been completed, return to the domain mapping area

Go to Tools > Domain Mapping

Revert the primary domain to the custom domain previously used.

NB: Whilst this workaround is in effect your custom domain will still work, however in the address bar shown in the browser when a person visits your website will redirect to the *.blogs.ilrt.org version.